1. Field in the Industry
The present invention concerns an anode with high activity, dimensional stability and no dissolution in prolonged operation in electrochemical reactions such as electrolysis and electrodeposition.
2. Prior Art
In general, titanium and other metals are used as the anode for electrolysis of solution of electrolytic substances and electrodeposition. Since the anode is polarized at high potentials and exposed to highly oxidizing condition, use of such metals is to utilize the characteristics of the metals that they form corrosion-resistant insulating film under such highly oxidizing condition.
However, the insulating surface film thus formed prevents the electron transfer through their surfaces which is the role of an electrode. In order to overcome this contradiction, titanium coated with oxides of an element or elements of the platinum group (hereinafter referred to as “platinum group element(s)”) resistant to highly oxidizing condition and having a high activity for the anode have been used.
For such electrodes, coherence of the electrocatalyst oxides of platinum group element(s) with the titanium substrate becomes effective if the electrocatalyst oxides have the same crystal structure as that of titanium oxide in addition to sufficient electric conductivity. Under such conditions the electrode will consist of a continuum of titanium substrate and electrocatalyst consisting of double oxide of titanium ion and electrocatalyst metal ion. The currently used dimensionally stable anodes are composed of the titanium substrate coated with oxides of the platinum group elements such as RuO2, RhO2, PdO2, and IrO2. These oxides have the same rutile structure as TiO2 and their lattice constants are not largely different from those of TiO2, with a consequent continuity from the titanium substrate to the electrocatalyst. Among these oxides IrO2 is regarded to be the most suitable.
However, the wide use of the precious metal electrodes will result in consumption of a large amount of precious metals, and will lead to the lack of the resources. Thus, the highly active electrode with the use of small amounts of precious metals is desirable.
It is desirable for the coating substance on the titanium substrate to have the same rutile structure as TiO2 and to be stable even under highly oxidizing condition. Tin oxide, SnO2, has the same rutile structure as TiO2 and is stable even in highly oxidizing environments. Thus, the inventors found use of SnO2 together with precious metal oxides. Electric conductivity of SnO2 is not high, and this is a difficulty in using. The inventors found that addition of Sb to Sn enhances electric conductivity of SnO2.